I've looked through the current development source code for FBReader and have backported all the changes which are described as bug fixes. I've uploaded a new version incorporating these fixes as 0.12.10-23 (for Diablo and Fremantle).

The descriptions given for the included changesets are:

• #330 has been fixed
• fixed ePub cover extracting
• LitResDataParser code fix
• Mobipocket image processing has been fixed
• text drawing optimization (actual for Chinese texts)
• ZLCharSequence bug has been fixed
• LZXDecompressor fix
• RtfReaderStream fixes

The first one is the one that seems to fix CSS handling - this looks to fix all the issues I've seen with CSS and user-overrides anyway.

The source code changes can be downloaded from here.